Thermal enclosures are used to keep the contents of the enclosure at a temperature other than ambient for as long a period as possible. A thermal enclosure is a container, one wall of which contains means for adding or removing heat from the interior of the container. For example, such means may include ice or materials which can be mixed to provide either endothermic or exothermic reactions. Such means may also include materials having a relatively high specific heat and/or high heat or fusion.
One material which is particularly adapted for use in a thermal enclosure is refrigerant gel. Refrigerant gel has very high specific heat and a very high heat of fusion. Moreover, it can be readily packaged and reused, and it does not liquify when its temperature exceeds its freezing point, i.e. that temperature where a quantity of heat energy is transferred without any resultant change in temperature due to a change of state. Refrigerant gel is disclosed, by way of example, in U.S. Pat. No. 2,803,115.
One prior art thermal enclosure uses packaged refrigerant gel in the cover of a container to maintain the contents of the container cool. The cover and the refrigerant gel can be cooled in a freezer and then used to maintain a reduced temperature within the container. The cover of this prior art enclosure is hollow and includes a main section and a bottom wall. The main section has a peripheral wall and an upper wall which define a downwardly opening cavity. With the cover inverted, packaged refrigerant gel can be inserted into the cavity of the main section of the cover. Thereafter, the bottom wall is adhesively attached to the main section of the cover to enclose the opening.
With this construction, the bottom wall is outside the cavity defined by the main section of the cover. The edges of the bottom wall are exposed, and during use the bottom wall tends to peel off. The tendency of the bottom wall to separate from the main section of the cover is augmented by the weight of the refrigerant gel acting downwardly against the bottom wall. Because of the tendency of the bottom wall to separate or peel off, it must be carefully glued to the main section of the cover and held under pressure while the glue cures. Even with these precautions, the problem with bottom wall separation cannot be completely remedied.